Vertical structure of the atmosphere

Review of last lecture Earth’s energy balance at the top of the atmosphere and at the surface. What percentage of solar energy is absorbed by the surface? Earth’s energy balance at the top of the atmosphere and at the surface. What percentage of solar energy is absorbed by the surface? Atmospheric influences on radiation (3 ways) Atmospheric influences on radiation (3 ways) The three types of atmospheric scattering. What causes the blue sky? Why causes the reddish-orange sunsets? The three types of atmospheric scattering. What causes the blue sky? Why causes the reddish-orange sunsets? What cause the greenhouse effect? What are the major greenhouse gases? Why is methane important? What cause the greenhouse effect? What are the major greenhouse gases? Why is methane important? Sensible heat flux (dry flux from warm to cold regions) and latent heat flux (wet flux from wet to dry regions) Sensible heat flux (dry flux from warm to cold regions) and latent heat flux (wet flux from wet to dry regions)

Atmospheric Thickness No defined top to the atmosphere No defined top to the atmosphere The atmosphere is very shallow—and is less than 2% of the Earth’s thickness The atmosphere is very shallow—and is less than 2% of the Earth’s thickness Over 90% of atmosphere in the lowest 16km & is where nearly all weather occurs

Temperature Basics Temperature – measure of average kinetic energy (motion) of individual molecules in matter Temperature – measure of average kinetic energy (motion) of individual molecules in matter Different temperature scales: Kelvin, Celsius, Fahrenheit Different temperature scales: Kelvin, Celsius, Fahrenheit  All scales are relative  degrees F = 9 ⁄ 5 degrees C + 32  degrees K = degrees C

Temperature Layers Longwave, Latent heat, Sensible heat Ultra-violet sunlight Solar winds, Cosmic rays

An artist’s view

Space shuttle Endeavour straddles mesosphere and stratosphere

Video: Ionosphere Troposphere and Mesosphere L1MD24 L1MD24 L1MD24 L1MD24

Pressure Essentials Pressure – force exerted/unit area (weight above you) Pressure – force exerted/unit area (weight above you) units - Pascals (Pa) or millibars (mb) (1 mb = 100 Pa) units - Pascals (Pa) or millibars (mb) (1 mb = 100 Pa) Average surface pressure over globe: mb. Average surface pressure over globe: mb. Atmosphere is mixture of gases -> partial pressure. Atmosphere is mixture of gases -> partial pressure. Dalton’s Law: sum of partial pressures equals total pressure Dalton’s Law: sum of partial pressures equals total pressure Pressure gradient (pressure difference between two locations/distance) gives rise to a force (pressure gradient force), which sets the air in motion. Pressure gradient (pressure difference between two locations/distance) gives rise to a force (pressure gradient force), which sets the air in motion.

The Equation of State (Ideal Gas Law) Describes relationships between pressure, temperature, and density (Start w/ molecular movement in sealed container  Pressure proportional to rate of collisions between molecules and walls). At constant temperatures, an increase in air density will cause a pressure increase (Add more molecules  increase density  increase rate of collisions  raise pressure) Under constant density, an increase in temperature will lead to an increase in pressure ( Raise temperature  increase speed of molecules  increase rate of collisions  raise pressure) Pressure = density x temperature x 287 J kg -1 K -1 [ p = ρ T R ]

 Pressure decreases with height Why? Because downward gravity force is balanced by vertical pressure gradient (called hydrostatic equilibrium) Δ p/ Δ z = ρ g Vertical pressure distribution: Hydrostatic equilibrium ρgρgρgρg Δ p/ Δ z

Pressure decreases non-linearly w/ height (Why? Because air is compressible, so denser near the surface) Δ p/ Δ z = ρ g Vertical pressure distribution (cont.)

Horizontal pressure distribution and horizontal pressure gradient Pressure maps depict isobars, lines of equal pressure Through analysis of isobaric charts, pressure gradients are apparent Steep pressure gradients are indicated by closely spaced isobars Typically only small gradients exist across large spatial scales (4% variation at continental scale), smaller than vertical gradients Surface pressure chart  H: high pressure L: low pressure

Simply determined by temperature: Warmer air has higher pressure in the upper level From the hydrostatic equilibrium, From the hydrostatic equilibrium, Δp/Δz=ρg So So Δz=Δp/(ρg) (1) Δz=Δp/(ρg) (1) This means that for the same amount of air, its thickness is determined by its density. From the equation of state ρTR P=ρTR So So ρ = P/(TR) (2) ρ = P/(TR) (2) This means that warmer air has a lower density This means that warmer air has a lower density Combine (1) and (2), we get: Δz=(Δp)TR/P This means that for the same amount of air, warmer air has a larger thickness (thermal expansion) Horizontal pressure distribution: Upper Air

Horizontal Pressure distribution: Upper Air Warm temperature leads to high pressure in the upper level Cold temperature leads to low pressure in the upper level Pressure difference drives wind to flow from high pressure to low pressure Low pressure at the surface High pressure at the surface

The most common atmospheric circulation structure L H H L Heating Cooling or No Heating Imbalance of heating  Imbalance of temperature  Imbalance of pressure  Wind

Summary Thickness of the atmosphere: less than 2% of Earth’s thickness Thickness of the atmosphere: less than 2% of Earth’s thickness Definition of temperature and its unit. Definition of temperature and its unit. Four layers of the atmosphere, what separate them? Four layers of the atmosphere, what separate them? Definition of pressure and its unit. Definition of pressure and its unit. Definition of pressure gradient. Pressure gradient sets the air in motion. Definition of pressure gradient. Pressure gradient sets the air in motion. Equation of state (Relationship between P, ρ, and T) Equation of state (Relationship between P, ρ, and T) Vertical Pressure Distribution. What is the hydrostatic equilibrium? How does pressure change with height? Vertical Pressure Distribution. What is the hydrostatic equilibrium? How does pressure change with height? Horizontal Pressure Distribution. Most common atmospheric circulation structure Horizontal Pressure Distribution. Most common atmospheric circulation structure